1. Field of the Invention
The present invention relates to an image forming apparatus which uses an intermediate transfer belt and a method thereof. More particularly, the present invention relates to an image forming apparatus which is capable of preventing a resistance of an intermediate transfer belt from varying by using a refresh charge supplied to the intermediate transfer belt.
2. Description of the Related Art
With the development of electronic technology, peripherals for computers, such as printers or scanners, are increasingly utilized. While printer manufacturers have invested in development of various printers, laser printers have risen to prominence in recent years. Laser printers have distinguishable advantages over from dot printers or ink-jet printers such has improved print quality, print speed, and noise reduction. Laser printers use a principle that attracts toners to a drum by using laser beams converted into an image signal and fuses the toners to paper with high temperature heats.
The laser printer prints an image through a series of processes, i.e., charging, writing, developing, transferring, and fusing. The charging process supplies a high voltage (approximately 7000V) to a charger and forms a negative (−) electric charge on a surface of the drum by a corona discharge. The writing process scans the surface of the drum, on which the negative (−) electric charge is formed, with laser beams and removes the negative (−) electric charge in the form of text, thereby forming a latent image. The developing process rotates a developing roller and the drum with a predetermined gap therebetween, thereby attracting toner particles having a negative (−) property to the latent image. The transferring process supplies a predetermined transfer voltage to a transferring medium when paper passes between the drum and the transferring medium and forms a positive (+) electric charge on the backside of the paper, thereby transferring the negative (−) toner particles formed on the surface of the drum to the paper. The fusing process applies heat and pressure to a toner image formed on the paper and fuses the toner image onto the paper. Through the above-described processes, an image is formed on the paper and output.
In color printers, toners having the four colors of cyan, magenta, yellow, black are used to obtain a color image. In order to obtain a more vivid color image, four photoconductors are used, and printing operations are performed by color toners. Also, the transferring process uses an intermediate transfer belt and performs two steps to output the respective color toners to appropriate positions.
FIG. 1 is a view showing a general image forming apparatus which performs a transferring process by using an intermediate transfer belt and through two steps. Referring to FIG. 1, the image forming apparatus comprises an intermediate transfer belt 10, a pre-cleaning roller 20, a driving roller 25, four first transfer units 30, 40, 50, 60, four respective photoconductors 35, 45, 55, 65, a second transfer backup roller 70, and a second transfer unit 75.
The driving roller 25 moves the intermediate transfer belt 10 at a proper speed, and the pre-cleaning roller 20 cleans off toners being left on a surface of the intermediate transfer unit 10 before the toners are removed by a cleaning blade 23.
The first transfer units 30, 40, 50, 60 correspond to the first to fourth photoconductors 35, 45, 55, 65, respectively, and the intermediate transfer belt 10 is located therebetween. Accordingly, the first transfer units and the photoconductors transfer their respective toners, each having one of black, cyan, magenta, and yellow, respectively, to the intermediate transfer unit 10. That is, toners having colors of black, cyan, magenta, and yellow are attracted to respective ones of the photoconductors 35, 45, 55, 65 during the developing process, and then transferred to the intermediate transfer belt 10 by the first transfer units 30, 40, 50, 60. In this case, the image forming apparatus recognizes location marks formed on the intermediate transfer belt 10 and synchronizes operations of the photoconductor 35, 45, 55 and 65 and the first transfer units 30, 40, 50, 60 so that one color image is formed.
Meanwhile, when the paper is conveyed in an arrowed direction as shown in FIG. 1, the color image is transferred from the surface of the intermediate transfer belt 10 to the paper 80 by a second transferring process, which is performed between the second transfer backup roller 70 and the second transfer unit 75.
FIG. 2 is a cross-section view showing the intermediate transfer belt 10. Referring to FIG. 2, the intermediate transfer belt 10 is comprised of a coating layer 11, a urethane rubber layer 12, and a base layer 13. The urethane rubber layer 12 and the base layer 13 contribute to the improved durability of the intermediate transfer belt 10, and the coating layer 11 contributes to the gain of a vivid color image. The first transfer units 30, 40, 50, 60 and the photoconductors 35, 45, 55, 65 are in close contact with upper and lower surface of the intermediate transfer belt 10, respectively, to transfer color toners to the intermediate transfer belt 10. For this, a transfer voltage is supplied between the first transfer units 30, 40, 50, 60 and the photoconductors 35, 45, 55, 65 to transfer the color tones to the intermediate transfer belt 10. In this case, a potential difference of approximately 500V to 600V is generated on the upper and lower surfaces of the intermediate transfer belt 10. When the first transferring process is completed, the transfer voltage is stopped. Thus, the potential difference on the upper and lower surfaces of the intermediate transfer belt 10 disappears in a normal case, that is, when no transfer process is occurring.
However, as the number of printed copies increases, electric charges existing in the urethane rubber layer 12 of the intermediate transfer belt 10 do not recombine to each other, which is referred to as a polarization effect. As shown in FIG. 2, the negative (−) electric charge in the urethane rubber layer 12 is aligned along a boundary “a” between the urethane rubber layer 12 and the coating layer 11, whereas the positive (+) electric charge is aligned along a boundary “b” between the urethane rubber layer 12 and the base layer 13. Therefore, although the transfer voltage is not supplied, a predetermined potential difference Vp is still generated in the upper and lower surfaces of the intermediate transfer belt. As a result of the polarization, a resistance of the intermediate transfer belt 10 increases.
FIGS. 3A and 3B are graphs explaining the polarization effect and the increase of the resistance which are caused as the number of printed copies increases. In detail, FIG. 3A is a graph showing a variation of a voltage measured at opposite ends of the intermediate transfer belt 10. FIG. 3A shows a first curve 310 indicating a variation of a voltage measured in an initial state of the image forming apparatus, and a second curve 320 indicating a variation of a voltage measured after the image forming apparatus outputs 5000 or more copies. According to the first curve 310, when a transfer voltage of 500V is stopped, a potential difference generated on opposite surfaces of the intermediate transfer belt 10 abruptly decreases to 0V. According to the second curve 320, although the transfer voltage of 500V is stopped, a potential difference of approximately 200V remains. A predetermined voltage still remains on the intermediate transfer belt 10 due to the polarization. Accordingly, if a transfer voltage having the same level as in the initial state is supplied, the transferring process is not normally performed.
FIG. 3B is a graph showing a relationship between a resistance and the number of printed copies. Referring to FIG. 3B, the intermediate transfer belt 10 has a resistance of approximately 25MO in an initial state. However, at the time of printing of thirty thousand copies, the resistance increases to 337MO, and at the time of printing of fifty thousand copies, the resistance increases to 474MO. When the resistance abruptly increases, the level of the transfer voltage must increase to maintain a transfer quality. As the transfer voltage increases, the polarization becomes more significant, and thus the resistance of the intermediate transfer belt 10 increases more rapidly. As a result, a lifespan of the intermediate transfer belt 10 is reduced, and power consumption increases.
Although the level of the transfer voltage increases to compensate for the variation of the resistance, proper conditions for the transferring process may not be present, thereby deteriorating image quality.
A need therefore exists for an image forming apparatus that overcomes the problems associated with polarization of the transfer belt.